Pump-up pump-down hydraulic jack



Jan. 8, 1963 R. w. BORN PUMP-UP PUMP-DOWN HYDRAULIC JACK Filed Oct. 31, 1960 2 Sheets-Sheet l INVENTOR. lam Mama W. BOP/V 7 Jan. 8, 1963 R. w. BORN PUMP-UP PUMP-DOWN HYDRAULIC JACK 2 Sheets-Sheet 2 Filed Oct. 51. 1960 INVENTOR.

ens Mama. 1 4 saw/v %1 @Q 12 02MB United States Patent Office 3,071,927 Patented Jan. 8, 1963 3,071,927 PUMP-UP PUMP-DOWN HYDRAULIC JACK Raymond W. Born, West Covina, Calif., assiguor to Hydraulic'Engineering, Inc., West Covina, Calif a corporation of California Filed Oct. 31, 1960,'Ser. No. 66,003 20 Claims. (Cl. 60-52) This invention relates to hydraulic jacks and more particularly to an improved hydraulic jack assembly designed to be pumped up and pumped down and having a unique automatic valving mechanism functioning to permit jack movement in increments between periods of jack dwell and in a manner assuring positive control of jack movement at all times.

A particular characteristic feature of the jack is the fact that both extension and retraction of the jack requires positive operation of means for pressurizing the fluid.

Hydraulic jacks as heretofore designed and built lack adequate provision for safeguarding against unexpected retraction under load with the result that it is hazardous for inexperienced persons to operate them. Furthermore, in many operating environments more than one jack is necessary to handle the load and there is need for simple, foolproof and inexpensive means for synchronizing the operation of the several jacks with one another Very complex synchronizing arrangements have been proposed heretofore for this purpose. These have now reached a rather satisfactory state of design from a functional and synchronizing standpoint, but are complex, bulky and costly to an extent placing them beyond the reach of the ordinary user. For an example, mention is made by way of illustration of the problem of installing and removing a currently popular sleeper cabin with respect to the cargo compartment of a light-duty pickup truck. Such sleeper cabins are constructed separately from the truck with the lower portion being designed to fit within the cargo compartment of the truck when properly supported for lowering into this compartment. When the sleeper unit is not required for use, it is desirable to have means for quickly and easily removing the sleeper in order that the truck may be employed for its usual purposes. Accordingly, there is need for simple, inexpensive, thoroughly reliable jacking means by which the sleeper unit may be installed and removed from the truck with safety with a minimum of labor.

To meet this and countless other similar needs, there is provided by this invention a hydraulic jack assembly making use of a plurality of independent jacks coupled through flexible fluid lines with a fluid reservoir incorporating a specially designed pump and valving mechanism by which the jacks are operated in positive out-of-phase synchronism. Control is assured by operating the jacks in small increments alternately to one another by means requiring the positive application of power to the fluid line irrespective of whether the jacks are being lowered or extended, the out-of-phase increments of movement of the jacks being so small as to be unobjectionable. The safety valving mechanism functions automatically in response to the pressure existing in the jacks to cut off back flow unless and until the operator applies a higher controlled pressure in a manner to open the valves. Furthermore, the pumping means provided for applying the control pressure is so designed that this pressure can be applied only until a predetermined quantity of fluid has passed.

Accordingly, it is a primary object of this invention to provide an improved inexpensive hydraulic jack providing positive assurance against uncontrolled retraction under load.

Another object of the invention is the provision of an improved hydraulic jack assembly having a unique valving mechanism so designed as to require the application of a controlled pressure to the valving mechanism to effect both extension and retraction of the jack.

Another object of the invention is the provision of a hydraulic jack required to be pumped up and pumped down in increments positively spaced from one another whereby it is impossible for the jack to move in either direction except in short separated steps.

Another object of the invention is the provision of a hydraulic jack assembly making use of a plurality of similar jacks featuring the use of unique automatic valve mechanism in cooperation with pumping means for the fluid and functioning to operate the jacks in out-of-phase synchronized relation.

Another object of the invention is the provision of a multiple-unit hydraulic jack assembly of simple and inexpensive design incorporating safety features and by which one man can handle heavy loads under positively controlled conditions and with safety to himself and to the property involved.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawings to which they relate.

Referring now to the drawings in which a preferred embodiment of the invention is illustrated:

FIGURE 1 is a combined schematic and longitudinal cross-sectional view through portions of a hydraulic jack assembly incorporating the features of this invention in the process of raising a load;

FIGURE 2 is a fragmentary cross-sectional View through the fluid reservoir and valving mechanism of FiGURE 1 showing the positions of the valves during the downstroke of the pump and contrasting the position of the parts with FIGURE 1 wherein the pump is operating in its upstroke;

FIGURE 3 is a view similar to FIGURE 2 but showing the relative positions of the valves during the upstroke of the pump but with the positions of the two reversing valves reversed as is necessary when lowering the load;

FIGURE 4 is a fragmentary cross-sectional View of one of the reversing valves taken along line 44 on FIG- URE 3; and

FIGURE 5 is a fragmentary view of a reversing valve on an enlarged scale and showing the positions of the two balls when the lower ball is fully seated.

Referring more particularly to FIGURE 1, there is shown a preferred embodiment of the invention designated generally 10 as ararnged for handling a sleeper cabin 11 while being elevated for installation on a pickup truck. Hydraulic jack assembly 10 as there shown includes a pair of identical jack units 12, 12' of suitable design, capacity and extension capability for handling load 11. Jack units 12, 12' are connected through independent flexible fluid lines 13, 13', leading to separate identical control valve mechanisms mounted in the top portion of a fluid reservoir 15.

It will be understood that each jack unit is provided with a separate fluid line and with its own control valve mechanism now to be described and shown as incorporated in the construction of fluid reservoir 15. As here shown, reservoir comprises a cast chamber having a wide mouth inlet 16 at its top normally sealed closed by a cover assembly 17 suitably sealingly secured thereto by cap screws or the like not shown.

Cover 17 has formed therein separate fluid passages 18, 18 for each of the jack units. These passages are connected between the fluid pump and the various valves in the manner shown in FIGURE 1. For example, extending downwardly from passage 18 is a fluid inlet ipe 20 having an inlet check valve housing 21 supported at its lower end provided with an inlet port 22. A check valve 23 is normally held seated against the inner end of port 22 by a light spring 24. Also opening downward from passage 18 is a return flow passage 25 having an enlarged lower end seating therein a check valve 26 normally held seated by a spring 27 held in assembled position by a pin 28 mounted crosswise of the lower end of passage 25.

The very important reversible valve assembly 30 is interposed between the outer end of passage 18 and the inlet to the flexible tube 13 leading to jack unit 12. Valve assembly 30 includes a generally cylindrical chamber 31 located partly in an inwardly projecting boss 32 of reservoir 15 and partly within the overlying portion of reservoir cover 17. Held seated in an annular recess between the opposite ends of chamber 31 is a valve seating ring 34 having an O-ring sealing gasket 35 about its periphery. Ring 34- is provided with a pair of frusto-conical valve seats 36 and 37 each adapted to seat an associated ball valve 38, 39. Lower ball 39 is urged toward seat 37 at all times by a light spring 40. Upper ball 38 and lower ball 39 are in contact at all times and under all operating conditions, upper ball 38 being spring biased toward seat 36 only at certain times and under certain conditions by a precision calibrated spring 42. Spring 42 is located in the upper end of chamber 31 and preferably has its upper end attached to a seating disc 43 integral with a plunger 45 slida bly supported in a bore 46 opening through the upper surface of cover 17. This plunger is spring biased against the upper end of chamber 31 by a stiff coil spring 48 having its lower end resting against valve seat ring 34. Cooperating with spring 42 in controlling the operation of ball valves 38 and 39 is a stop pin 50 forming an axial extension of plunger 45 'with its lower end accurately positioned relative to ball 38 and its seat 36 in respects to be explained in detail presently.

Referring now more particularly to FIGURE 4, it is pointed out that plunger 45 has its upper end extending into the path of a cam lever 52 pivotally supported by pin 53 between lugs carried by the outer side of cover 17. Cam lever 52 is provided with a pair of flat surfaces 54, 55 interconnected by a rounded cam surface 56 and desirably spaced 90 degrees from one another. When cam lever 52 is in the position shown in FIGURE 4, flat 55 bears against the upper end of plunger 45 to depress the plunger in opposition to spring 48. On the other hand, when cam lever 52 is pivoted to an upright position, fiat 54 then bears against the end of plunger 45 under which conditions stiff spring 49 supports flange disc 43 and the valve limit stop 56 in the upper of its two operating positions. In the reversible valve design here shown, the permissible movement of plunger 45 and of limit stop 50, as permitted by flats 54, 55 of cam lever 52 is approximately 100 mils. In this connection it will be understood that these and other specific values mentioned below are given by way of example of values found to provide an excellent working relationship of the parts.

Other dimensions and relationships of the reversible valve are of importance as will now be explained. Thus, when cam lever 52 is adjusted to the position shown in 4 FIGURE 4, the lower end of limit stop 50 is spaced within a range of 20 to 30 mils above ball valve 38 when the latter valve is seated against seat 36.

The relative values and design characteristics of springs 40 and 42 are also of importance. In this connection, it is pointed out that lower spring :8 is relatively light and commonly characterized as a low-rate spring requiring the application of a five-pound load to compress it /a inch. Upper spring 42, on the other hand, is known as a high-rate spring and requires the application of 15 pounds to compress it A; inch. It is also pointed out that when cam lever 52 is pivoted to its upright position to bring flat 54 against the end of plunger 45, the lower end of spring 42 is then spaced out of contact with ball 38 to assure that lower spring 46 will be free to seat lower ball 39 without interference from spring 42.

An important aspect of the described jack assembly is the provision of a variable capacity chamber 60 in communication with passage 18 and capable of varying the pressure conditions in the latter passage as necessary to operate the reversible valve device as well as the two check valves. Chamber 60 communicates with passage 18 and is formed within a cylindrical extension 61 upstanding from cover 17 Slidably supported within chamber 60 is a piston 62 having a tubular operating rod 63 projecting upwardly therefrom and through a packing gland 65 to an operating hand grip 66 suitably secured to its upper end, as by nut 67. Ports 68 at the lower end of tube 63 communicate with the portion of the chamber above piston 62 and are vented to the atmosphere at the outer end of the handle in any suitable manner.

It will be evident from the foregoing that the reciprocation of piston 62 within chamber 60 is effective to vary the capacity of this chamber to force fluid contents therein into passage 18 or, in the alternative, to receive fluid contents from this passage depending upon the direction of reciprocation of piston 62.

Rigidly connecting piston 62 to a second and similar piston 62' located Within the variable capacity chamber 69' is a connecting rod or tube 63'. The latter passes through a packing gland and separating partition 70 formed in cover 17 and separating chambers 60, 60' from one another. Chamber 60' is formed by a cylindrical tube 61' in axial alignment with upper tubular housing 61 and extends vertically through reservoir 15. The upper end of tube 61 is sealingly seated within a downwardly opening well 72 formed in the underside of cover 17 and its lower end is seated within a well 73 in the bottom of the reservoir. The bottom of this well opens into a filling opening 74 and is normally closed by a fluid-tight removable plug 75. The lower end of tube 60' communicates with the interior of the reservoir through ports 77.

Inasmuch as both jacks 12 and 12 are provided with identical fluid control valving mechanisms responsive to changes in fluid pressure condition within the variable capacity chamber associated with each, the corresponding parts have been designated with the same reference characters but are distinguished from one another by the presence or absence of a prime, the unprimed numerals being associated with one jack unit and the primed numerals being associated with the operating components of the other jack unit.

The operation of the described pump-up, pump-down synchronized jack assembly is as follows. Referring first to FIGURES 1, 2 and 4, let it be assumed that jack units 12, 12 are arranged along the opposite sides of a sleeper cabin 11 with their bases supported on a suitable level footing. If it is desired to elevate cabin 11, cam levers 52, 52 of reversible mechanisms 30, 30' are elevated to the upright position with flat 54 resting against the top of plunger 45. Under these conditions stiff springs 48 hold the limit stop 50 in its upper position. The lower end of calibrated spring 42 is then out of contact with upper ball 38 and lower ball 3? is closed aaginst its seat 37.

It will therefore be understood that, in the closed position of ball 39, spring 42 exerts no pressure on ball 38. Accordingly, and under these conditions, spring 40 is effective to seat lower ball 39 and positively prevent back flow of fluid from jack 12 through conduit 13 into passag 18. The same is true respecting ball 39 of the reversible valve mechanism 30' associated with the other jack unit 12'. I

All parts of the several fluid passages being filled with fluid including variable capacity chambers 60, 60, let it be assumed that the operator grasps handgrip 66 of the pump and exerts an upward pull on piston rod 63. The resulting upward movement of piston 62 within chamber 60 acts to increase the volume of this chamber. No liquid can flow past closed ball 39 into passage 18 with the result that the decrease in pressure in passage 18 is communicated through tube 20 into intake valve housing 21 where it is effective to open intake valve 23. Fluid from reservoir 15 then enters port 22, passes upwardly through tube 20 into passage 18 and flows into chamber 60 filling the latter as the piston 62 continues to rise.

While the foregoing action is taking place with respect to chamber 60, a quite different action is occurring in the similar chamber 6t), passage 18', and jack 12. Thus, at the start of the upward stroke of piston 62, piston 62' is in its lower position and its associated chamber 60' is filled with liquid. Accordingly, as piston 62' acts to decrease the size of chamber 60' the liquid therein is forced into passage 18'. Inasmuch as spring 27 of the return valve assembly 26' is stronger than spring 40 of the reversible valve assembly, liquid displaced from chamber 60 is forced past open ball 38 and against lower ball 39 to open the latter permitting the pressurized fluid to flow along hose 13' into the lower end of jack 12 to extend the latter upward by a short increment directly related to the volume of liquid displaced from chamber 69'. Owing to the relatively large flow area available upon the slight depression of ball 39', this ball does not normally open sufficiently to permit seating of upper ball 38. Accordingly, jack 12 continues to receive fluid so long as pump rod 63 continues on its upward stroke during which time jack 12 remains stationary or in a dwell position.

Referring now to FIGURE 2 it will be seen that upon the reversal of the pump stroke, pistons 62 and 62 will move downward in their respective chambers 6t 60' at which time jack 12 will remain locked in a stationary or dwell position as jack 12 moves upwardly through a short increment. At the instant the pump rod ceases its upward movement the pressurized fluid in each jack 12, 12' is communicated back through their respective hoses 13, 13' and acts to hold both balls 39, 39' positively seated thereby preventing any back flow of fluid from either jack. At the beginning of the described downward stroke, upper chamber 60 decreases in volume as lower chamber 611 increases in volume. Fluid displaced from upper chamber 60 is then forced into passage 18 and into the reversible valve assembly 30 forcing ball 39 open so that the pressurized fluid can flow to jack 12 to extend it. The high pressure fluid in passage 18 does not flow back into reservoir 15 since spring 40 is so designed as to permit ball 39 rather than check valve 26 to open. The increase in volume of lower chamber 60 resulting from the downward movement of piston 62' reduces the pressure in passage 18' causing valve 23 to open to pass fluid from the reservoir into passage 18 and thence into chamber 60' in the same manner described above in connection with chamber 68.

It will therefore be evident that during the repeated to-and-fro movement of piston rods 63, 63' in their respective variable capacity chambers 60, 60', liquid is alternately forced into first one jack unit and then the other as the alternate variable capacity chambers are being refilled with a new charge of fluid. Accordingly, first one jack and then the other is extended by a small increment while the other jack remains locked in a stationary position. It will therefore be recognized that the jacks operate in out-of-phase relation characterized by a period of dwell followed by a period of movement.

Assuming that the jacks have been extended to any height desired, it will be apparent that they will remain locked in that position so long as no movement is applied to the operating handle to vary the capacity of the variable capacity chambers. Throughout such periods both ball valves 39, 39 are held positively seated by the pressure existing within each jack unit.

Let it now be assumed that the operator wishes to lower load 11. Cam levers 52, 52' are first pivoted through a degree arc to the position shown in FIG- URE 4 wherein their associated flats 55, 55' depress plungers 45, 45' to their lower positions shown in FIG- URES 3 and 4. Under these conditions, upper balls 38, 38' are seated against their respective seats 36, 36 under the very substantial pressure exerted thereon by stiff springs 42, 42 and lower balls 39, 39' are held in open position. The lower end of limiting stops 50, 50' are then spaced approximately 20 to 30 mils above closed balls 38, 38. This spacing i suflicient to permit lower ball 39 to seat against its seat 37 should the pressure in passages 18 become sufficiently low as to endanger sudden lowering of the associated jack unit. In other words, under normal operating conditions the spacing between ball 38 and limit stop 50 permits balls 38 and 39 to move to an intermediate position while a normal jack operating flow of liquid is taking place back into the reservoir from the jack unit while being responsive to an excessive back flow to seat and prevent further lowering under emergency conditions. By this expedient, a positive safeguard is provided against too rapid or unexpected lowering of the jack unit.

Let it now be assumed that the operator lifts handle 66 causing plungers 62, 62 to move upwardly from the position shown in FIGURE 3. As this occurs, the pressure in chamber 66 will be reduced as the volume of this chamber increases. In consequence, the reduced pressure therein together with the high pressure existing in conduit 13 is effective to open ball 38 allowing lower ball 39 to partially close. Accordingly, fluid from the jack flows past these partially open valves into the continually increasing volume of chamber 66 During this same portion of the upstroke, liquid trapped in lower chamber 68' will be forced into passage 18' forcing return valve 26' to open and permit the fluid to flow back into the reservoir. The flow cannot take place to jack 12 because its control ball 38' is now seated.

During the downward stroke of piston rod 63, 63 the reverse operation occurs and fluid trapped within jack 12 flows upwardly past balls 38', 39 and into chamber 6%) as liquid trapped in upper chamber 60 is forced past check 26 into the reservoir.

While the particular pump-up pump-down hydraulic jack herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.

I claim:

1. A self-contained hydraulic power assembly having its own charge of operating fluid sealed therewithin comprising a plurality of hydraulic cylinder devices, a fluid reservoir, separate conduit means extending between said cylinder devices and said reservoir and including pressure responsive valve means and a variable capacity chamber in the separate conduit means extending to each cylinder device, means for varying the capacities of said chambers alternately to operate the associated pressure responsive valve means alternately thereby to move one of said cylinder devices while the other remains substantially stationary thereby to change the positions of said cylinder devices alternately by successive small increments.

2. A synchronized hydraulic power assembly provided with its own charge of operating fluid confined for flow therewithin having a plurality of cylinder devices coupled to a plurality of fluid pumps and valving mechanisms which valves and pumping mechanisms are arranged to be operated in out-of-phase relationship with one portion of said cylinder devices receiving fluid under pressure while the other portion of said cylinder devices remain stationary and for thereupon reversing these conditions so that fluid under pressure is then delivered to the stationary cylinder devices to extend the same as the other cylinder devices remain stationary whereby said cylinder devices are operated sequentially in out-of-phase relation.

3. In combination, a hydraulic jack system having a pair of jacks operating in parallel against a load and adapted to be charged With a quantity of operating fluid confined therewithin for repeated reuse in said jack system, a fluid reservoir and conduit means extending therefrom to each of said jacks and having automatic valve means therein operable in response to pressure conditions in said conduit means to control the flow of fluid from and to said pair of jacks, and fluid displacement means connected to said conduit means operable in cooperation with said valve means to vary the quantity of fluid present in first one and then the other of said pair of jacks while maintaining the fluid condition of the other jack substantially unchanging whereby the extended condition of said jacks is sequentially changed in alternate increments.

4. The combination defined in claim 3 characterized in the provision of means for positively coordinating the operation of said jacks to vary the quantity of fluid change in said jacks in a predetermined fixed ratio.

5. The combination defined in claim 3 characterized in the provision of means for reversing the action of said valve means whereby the operation of said fluid displacement means is effective to reverse the direction of fluid flow in said conduit means to effect thereby a reverse movement of said jacks.

6. The combination defined in claim 3 characterized in that said conduit means requires but a single fluid flow channel between each jack and said fluid reservoir.

7. The combination defined in claim 3 characterized in that said means for varying the pressure conditions in said conduit means comprises a pair of pumps operating in independent fluid circuits connected to separate ones of said jacks, and means for operating said pumps in unison with one another.

8. The combination defined in claim 7 characterized in that said pumps and said valve means and said fluid reservoir comprise a unitary assembly.

9. The combination defined in claim 7 characterized in that said pair of pumps comprises a pair of similar cylinders supporting similar pistons therein interconnected by rigid piston rod means and including means extending from the latter for reciprocating said pistons and interconnecting rod means in unison.

10. The combination defined in claim 3 characterized in that said valve means comprises separate pairs of opposed movable valve means in separate conduit means extending between each jack and said fluid reservoir, said opposed valve means being in axial alignment and disposed on the opposite sides of a valve seat structure therebetween, means always effective to bias one of said pair of valve means toward its seat structure, and means selectively movable into and out of a position wherein it is effective to bias the other one of said pair of valve means toward its seat structure.

11. A pump-up pump-down pack assembly comprising a fluid reservoir, a fluid jack, conduit means connected between said jack and reservoir and a check valve controlled inlet pipe opening below the fluid level in the reservoir and a check valve controlled relief overflow port opening into said reservoir, a variable capacity fluid chamber opening into said conduit means including means for forcibly varying the capacity of said chamber to apply pressure to fluid in said conduit means, and reversible automatic valve means disposed in said conduit between said jack and said first check valve operable in response to pressure changes in said variable capacity chamber to control the direction of fluid flow to and from said jack in response to the alternately changing capacity conditions in said chamber whereby, when said reversible valve means is in one of its two positions, alternately increasing and decreasing the capacity of said chamber forces fluid into said jack and whereby, when said reversible valve means is in the other position thereof, alternately increasing and decreasing the capacity of said chamber permits fluid trapped in said jack to flow back to said reservoir.

12. A jack assembly as defined in claim 11 characterized in the provision of means for shifting said reversible valve means selectively between the two operating positions thereof.

13. A jack assembly as defined in claim 11 characterized in the provision of a plurality of jacks, fluid conduits, variable capacity chambers and reversible valve means all interconnected to a common fluid reservoir, means for varying the capacities of said chambers in out-of-phase relation to one another whereby to operate the respective jacks in out-of-phase relation with one jack remaining stationary while another is in motion.

14. A jack assembly as defined in claim 13 characterized in the provision of common means operable to vary the capacities of a pair of said chambers in reverse order and in predetermined coordinated relation to assure operation of the associated jacks in out-of-phase relation.

15. A jack assembly as defined in claim 14 characterized in that said common operating means for said chambers comprises manually reciprocable means movably supported for to-and-fro movement.

16. A jack assembly as defined in claim 13 characterized in that said variable capacity chambers comprise cylinders having pistons supported therein for to-and-fro movement axially of said cylinders.

17. A jack assembly as defined in claim 11 characterized in that said reversible valve means comprises a pair of balls operating in contact with one another and disposed on the opposite sides of an annular seating ring disposed therebetween and so arranged that the one or the other ball can seat but not both at the same time, means always biasing one ball toward seating position, and overriding means selectively operable to bias the other ball positively seated in opposition to said one ball and its biasing means.

18. A jack assembly as defined in claim 17 characterized in the provision of means for limiting the opening movement of said other ball valve when the biasing means therefor is positioned to override the biasing means for said one valve whereby a pressure condition in said conduit means effective to open said other valve is prevented by said limiting means from allowing said one ball valve from seating to cut off fluid flow.

19. A reversible valve assembly for use in a pump-up pump-down jack assembly of the type having a fluid jack connected to a reservoir via said reversible valve assembly and a variable capacity pressure chamber in communication with the fluid connection between the jack and reservoir, said valve assembly comprising a valve housing having a double-ended valve seating ring between the ends thereof which ends are adapted to be connected in series with the jack and fluid reservoir respectively, a pair of balls in contact with one another at all times and located on the opposite sides of said seating ring, spring means urging one ball toward its seat, movably supported spring means on the far side of the second ball from said first spring, said movable spring being eflective in one position thereof to urge the second ball against its seat in opposi- References Cited in the file of this patent UNITED STATES PATENTS 268,295 Zeigler Nov. 28, 1882 10 Nelson Oct. 5, Meixner Mar. 21, Boisset June 6, Kerber Feb. 18, Henry Dec. 28, Nilson June 27, Keeler Feb. 16, Kirkland June 27, Bill Mar. 7, 

1. A SELF-CONTAINED HYDRAULIC POWER ASSEMBLY HAVING ITS OWN CHARGE OF OPERATING FLUID SEALED THEREWITHIN COMPRISING A PLURALITY OF HYDRAULIC CYLINDER DEVICES, A FLUID RESERVOIR, SEPARATE CONDUIT MEANS EXTENDING BETWEEN SAID CYLINDER DEVICES AND SAID RESERVOIR AND INCLUDING PRESSURE RESPONSIVE VALVE MEANS AND A VARIABLE CAPACITY CHAMBER IN THE SEPARATE CONDUIT MEANS EXTENDING TO EACH CYLINDER DEVICE, MEANS FOR VARYING THE CAPACITIES OF SAID CHAMBERS ALTERNATELY TO OPERATE THE ASSOCIATED PRESSURE RESPONSIVE VALVE MEANS ALTERNATELY THEREBY TO MOVE ONE OF SAID CYLINDER DEVICES WHILE THE OTHER REMAINS SUBSTANTIALLY STATIONARY THEREBY TO CHANGE THE POSITIONS OF SAID CYLINDER DEVICES ALTERNATELY BY SUCCESSIVE SMALL INCREMENTS. 